The contributions of rods and cones to visual sensitivity will be examined by psychophysical experiments in the near peripheral retina. Central to our research is the hypothesis that the mechanisms mediating steady-state light adaptation and long-term dark adaptation are non- equivalent for stimuli possessing different spatial or temporal characteristics. Our analysis is based upon Stiles' two-color increment-threshold technique and a new metric, the d-function, which provides an estimate of the sensitivity of cones relative to rods. We will compare the effects of square-wave test stimuli and equal-energy temporal trapezoids (ramp functions) upon spatial summation. On- response functions will be carried out on ramps and square waves to test an amplitude vs a latency hypothesis of rod-cone interaction. The summative properties of the underlying mechanisms will be further examined by measuring sensitization functions with ramps or square waves.